Why does it matter who has the page on watchlist? One can also get the "contagion" looking at recent changes or stumbling upon the change while reading the page, or while looking at a subsequent diff e.g. by a vandal. Also, unless you plan to try a contagion yourself (not a good idea), you'll be looking at past data, but we don't have the watchlists for the past. Moreover, you'd need to find a way to establish a causation connection between two similar edits in any case, or they could just both caused by a coordinated effort like a policy change or a cleanup sprint or a collaboration between two users. Speaking of which, an edit (especially if repeated on multiple pages) can also cause "retortion" edits in opposite direction on other articles, or policy changes in either direction, so the "contagion" may not be caused directly but rather by causing a coordinated effort, spreading either the virus or its antivirus.
Nemo